Famvir (famciclovir) is an antiviral medicine used for the treatment of various herpesvirus infections, including herpes zoster, genital herpes and herpes labialis (cold sores). It is a prodrug form of penciclovir with improved oral bioavailability. Famvir is available as 125 mg, 250 mg, and 500 mg tablets. The dose of Famvir will vary depending on the treatment of shingles or genital herpes.